In time-free environments human circadian rhythms usually free-run with a period of about 25 hrs. In natural conditions the circadian rhythms are entrained (synchronized) by the 24-hr Zeitgebers (time-givers). There are many putative Zeitgebers for man, such as the natural light-dark (LD) cycle, social cues and the scheduling of sleep and wake. Recent experiments suggest that only sunlight (or very bright artificial light), but not ordinary indoor light, might be an effective Zeitgeber. Humans in isolation units have been entrained by non 24-hr Zeitgebers such as the signals to go to bed and wake up. The range of entrainment of the temperature rhythm was usually from about 23 to 27 hrs. Recently, the upper limit of entrainment has been extended past 29 hrs by a LD cycle with very bright light. These studies will explore the theoretical and practical applications of non-24-hr sleep-wake schedules and bright light. In all studies rectal temperature will be continuously measured. Subjective sleep parameters and mood will be assessed daily. The first study will determine whether the temperature rhythm can be entrained to 26 or 23.5-hr sleep-wake schedules while the subjects live at home exposed to the competing 24-hr Zeitgebers. In one condition there will be a bright light Zeitgeber (2 hrs every 26 or 23.5 hrs) plus provisions to block out the natural 24-hr LD cycle. If these light manipulations facilitate entrainment, they will be used in the next two studies. The second study will evaluate these sleep-wake schedules as treatments for people with the delayed sleep phase syndrome (DSPS) i.e., extreme "night-owls." Subjects will follow a 23.5 or 26-hr schedule (chronotherapy) until the sleep period reaches an earlier "target" time. The third study will evaluate a sleep-wake schedule designed to improve the sleep of nurses who work rotating shifts. There will be two identical one-month work schedules, each containing a transition from the day to the night shift and a transition back to the day shift. During the first month the nurses will follow their usual sleep schedule. During the second month they will follow a 26-hr schedule to aid in the transitions between shifts.